This invention relates to servo control systems of the type used to control the motion of a movable member along a path from a starting point to a destination over repeated cycles of movement. More particularly, this invention relates to servo control systems of the dual mode type.
Dual mode servo-systems are known and have been employed in the past to control the motion of a relatively movably member, such as a read/write transducer carriage assembly in a rotating disc drive unit, a rotatable printing member such as a cylindrical type element or a daisy wheel print element, a translatable carriage in a daisy wheel printer and other objects capable of being electromechanically driven. Dual mode servos are so termed since the control circuitry operates in two different modes of operation during a motion cycle, with the paired modes being variously called coarse and fine, non-linear and linear, velocity and position, and other designations used by those skilled in the art. Essentially, in one mode of operation the motion of the driven member is controlled in accordance with the instantaneous velocity of the member provided by a feedback signal from an appropriate transducer mechanism coupled to the member until the driven member is within a final portion of the motion path close to the desired destination. When the final portion of the motion path is entered, the control circuitry switches to the alternate mode of operation in which a linear position signal also generated by an appropriate transducer coupled to the member is added to the control feedback circuitry as the member approaches its final destination. Once the destination is reached, the alternate mode of operation is maintained so that the driven member is stabilized over the destination position. When a new destination is called for, typically by supplying a destination information signal to the control circuitry electronics, the dual mode servo switches back to the first mode of operation and is controlled in accordance with the velocity signals until the motion path portion is entered, whereupon the control circuitry switches in the linear position signal and the servo circuit is again operated in the alternate mode. This dual mode of operation is cyclically repeated as subsequent destination information signals are presented to the system.
In nearly all known dual mode servo-systems, the desired objective is to quickly move the driven member from the starting position to a position closely adjacent the final destination position and then to home in to the destination position in a more precise fashion. The former is accomplished during the mode of operation termed coarse, velocity or non-linear, while the latter is accomplished during the second mode of operation termed fine, position or linear. During the first mode of operation, the driven member is quickly accelerated to the maximum design velocity and then the velocity of the member is reduced typically in a step-by-step manner by comparing the actual velocity of the driven member with a predetermined velocity profile generated by a control circuit and controlling the electrical power applied to the electrical drive motor in accordance with the difference between the magnitudes of these two signals. The velocity profile is typically determined in advance and usually comprises a velocity step function whose magnitude is dependent upon the distance of the member from the actual destination. In addition, the velocity step function usually diminishes with decreasing distance between the instantaneous position of the member and the specified destination.
When the servo circuitry is switched to the fine mode, the desired velocity profile either falls to zero or is switched completely out of the control circuitry so that the motion of the member is controlled along the final path portion by a signal which is the algebraic difference between the actual velocity signal and the actual position signal, the latter typically being a sinusoidal or triangular signal designed to be substantially linear when the member is close to the destination position.
As noted above, the actual velocity and position signals, which are representative of the actual instantaneous velocity and actual instantaneous position of the driven member, are typically generated by transducers which are mechanically coupled to the driven member and which are capable of generating electrical signals representative of or proportional to the actual velocity and actual position of the driven member. Many types of such transducers are known: In some applications, separate transducers are employed to generate the velocity signals and the position signals. For example, a tachometer mechanically linked to the motor drive shaft is typically used to generate the velocity feedback signal; while an optoelectronic position transducer consisting of one or more light sources and a corresponding number of light receptors, a fixed reference aperture and a movable grating having alternating translucent and opaque indicia, is typically used to generate the position signals. In other applications, the actual velocity signals are derived from the position signals, either by differentiating a single cyclic analog position signal, or by commutating a plurality of space phased cyclic position signals and differentiating the commutated segments thereof.
Known dual mode servo-systems of the above-noted type have been found to suffer from several disadvantages when employed in certain applications requiring precise positioning of the driven member in the shortest possible period of time between the beginning and the end of a motion cycle. For example, in many dual mode servo-systems, the driven member will overshoot the destination position during fine mode operation and will describe dampened oscillatory motion about the destination position. Given a sufficiently long period of time, the driven member will be electronically stabilized over the destination position: However, in some applications such as printing applications in which the servo is used to control the position and motion of a rotatable print element the desideratum is to stop the motion of the driven member as quickly as possible, perform a function (e.g., printing) whose successful operation is dependent upon stabilization of the driven member, and proceed with the next motion cycle as quickly as possible. For example, in an application in which the servo circuitry is used to control the rotation of a daisy wheel, the design criteria employed for the printing control circuitry must allow for print wheel settling time during which the print wheel is describing damped oscillatory motion about the detent or destination position. It the allowed maximum settling time is selected to be too small, the print wheel will still be describing motion of sufficient amplitude to cause a blurred character to be printed; on the other hand, if the settling time is selected to be too long, the character printing speed will be inherently slow, which is highly undesirable. Although empirical adjustments can be made to the servo circuitry to reduce the amount of overshoot and also the settling time, the fact that the control signals are derived from the actual velocity and position of the print wheel are a limiting factor in minimizing overshoot and settling time.
Another disadvantage inherent in many known prior art dual mode servo-systems is that of electrical noise generated in various portions of the system. For example, in those servo-systems in which the velocity signals are derived from the position signals, noise signals are generated by the commutation and differentiation circuitry, which signals introduce signal errors into the circuitry with the result that the electrical signals which are intended to be truly representative of the actual instantaneous velocity of the driven member contain errors. Since the velocity signals deviate from the true velocity of the driven member, which errors cannot be accurately predicted and thus compensated for, and since these velocity signals are compared with the desired velocity profile signals to produce control voltages for driving the motor used to power the driven member, the driven member is not operated in accordance with the true design criteria of the system, as a consequence of which system performance suffers.